1. Field of the Invention
The present invention relates to an electronic still camera in which an object is photographed by image pickup means such as a solid image pickup element and the video signal of the object is recorded into a rotary recording medium such as a magnetic disc or the like, and, in particular, to the control of exposure of such electronic still camera.
2. Description of the Related Art
Recently, there has been developed an electronic still camera system in which an image pickup device such as a solid image pickup element, an image pickup tube or the like is combined with a recording device employing as a recording medium thereof an inexpensive magnetic disc having a relatively larger storage capacity such that an object can be still photographed electronically and recorded into a rotating magnetic disc and the reproduction of the recorded image can be performed by means of a television system, a printer or the like which is provided separately from the electronic still camera system.
In the electronic still camera of the above-mentioned type, as in a so called conventional silver salt still camera (which uses a silver salt film), a single lens reflex camera open photometric system is used because it provides easy framing, easy measurements of distances and so on. In the silver salt still camera, a spring adapted to energize and rotate a clip washer for moving aperture blades is previously charged and engaged; the engagement of the spring is removed according to a release operation so as to stop down the diaphragm momentarily; and, after exposure, when a film is wound up, the spring is charged again.
In such electronic still camera, as described before, there is employed as image pickup means a solid image pickup element such as a charge coupled device (CCD) or the like. For this reason, if the electronics still camera is provided with a function to accumulate electric charges, for example, on the image pickup surface of CCD according to the brightness of an image of an object in photographing, transfer the accumulated electric charges in recording, and discharge remaining electric charges in preparation for the next photographing, then basically there is eliminated the need of a mechanical shutter that is used in the ordinary silver salts still camera.
As the conventional examples of the electronic still camera that is equipped with the above-mentioned electronic shutter function, Japanese Patent laid open No. 62-38675, 62-48182 and 62-172488 can be cited. In Japanese Patent laid open No. 62-38675, there is disclosed an electronic still camera which is provided with a basic electronic shutter function to discharge the remaining electric charges of a solid image pickup element, then pick up an image, and further read out accumulated electric charges.
Also, in Japanese Patent laid open No. 62-48182, there is disclosed an electronic still camera equipped with an electronic shutter function which coincides the time of termination of accumulation of signal charges (which agrees with a read-out time) in a solid image pickup element with the output timing of a vertical synchronization signal.
Further, in Japanese Patent laid open No. 61-172488, there is disclosed a solid image pickup device comprising a light receiving section and an accumulation section, in which electric charges accumulated for a long period of time (about 33 m sec) in the accumulated section is transferred at a high speed of a short time (.DELTA.t) and only the electric charges accumulated in this short time are taken out as video signals, so that the shutter effect of the exposure time .DELTA.t can be obtained.
As conventionally, if an electronic still camera is structured such that its photographing operation is performed only by means of the above-mentioned electric processings (that is, the accumulation, transfer and discharge of the electric charges in the solid image pickup element) while the image pickup surface of the solid image pickup element such as CCD or the like being exposed, all of unnecessary electric charges cannot be discharged out after photographing, which results in the lowering of the quality of a recorded image (a reproduced image) is similarly lowered in quality.
Also, in the electronic still camera that is provided with only the above-mentioned electronic shutter function as the shutter function thereof, there arises a problem of so called "smearing", that is, while signal charges accumulated in the solid image pickup element are being read out, charges due to the light continuously falling on the light receiving surface of the solid image pickup element are applied to the signal charges being transferred, so that false signals are detected, resulting in the lowering of quality of images.
In order to minimize the influences of the smearing that is produced when an object is photographed by the electronic still camera provided with only the electronic shutter function, the signal charges accumulated in the solid image pickup element must be transferred to a transfer section at a high speed, that is, the signal charges must be read out at a high speed.
However, if the signal charges are transferred at a high speed, there arises an antinomic problem, that is the transfer efficiency of the signal charges in the solid image pickup element is lowered and, as a result of this, the quality of the image is also lowered.